Biomedical Engineering Bridging Medicine and Technology

Prelude

The cell is the basic functional unit in the body. The human body is composed of more than 200 different types of cells (Figure 5.1). Each cell of an individual is genetically the same: They all share the same genetic information, but cell types within an individual differ with respect to size, shape, and constituent molecules (Figure 5.2); therefore, they have different properties. For example, liver cells have abundant enzymes for detoxification of chemicals whereas red blood cells have abundant hemoglobin for oxygen transport. These differences are important to the function of the cell in the context of the organ in which it resides.

The human body contains trillions of cells: most estimates range from 10 to 200 trillion cells in an average person, but a research group examining all of the estimates suggests that 37 trillion is the best number (1). Despite a great diversity of cell composition and function, the trillions of cells in each person share common properties. In addition to their identical genetic material, most cells in the body have the same structural organization. Cells in humans are all surrounded by a lipid bilayer membrane and contain many of the same structural features. For that reason, biologists can discuss general principles of the operation of the cell. Of course, specific types of cells exhibit behaviors and properties that are specific to that cell type.

Much of what is known about cells has been learned from the study of cells in culture (Figure 5.1). Cell culture, or the maintenance and growth of cells outside of the body, was first discovered in the early 1900s and is now routine. The ability to maintain and study cells in a controlled and reproducible fashion has enabled scientists to dissect and classify many of the functions of human cells. Experiments in cell culture are often called in vitro experiments—from the Latin phrase meaning “in glass”—because they occur in an artificial environment, outside of the living organism.